1. Field of the Invention
The present invention relates to a method for the electrochemical machining of at least one workpiece, in which the workpiece is treated in different phases with a current flow which has different current densities during the different phases.
2. Description of the Related Art
Electrochemical machining (ECM) is a method for removing metal by electrochemically detaching the metal from the surface of an anodically polarized, metallic workpiece, which is a component part of an electrochemical cell. The method is based on the controlled, anodic, electrochemical degradation of the surface of the metallic workpiece to be machined (anode) with a tool (cathode) in the electrochemical cell filled with an electrolyte, by electrolysis. Electrolysis is a chemical process in which an electric current flows between two electrodes (the positively charged anode and the negatively charged cathode) immersed in a solution, the electrolyte.
Typical examples of an anodic, electrochemical degradation of the surface of a metallic workpiece to be machined are electrochemical polishing (ECP), electrochemical deburring (ECD) and the already mentioned electrochemical machining. During electrochemical polishing, the surface of the metallic workpiece is removed. Electrochemical machining is often performed in different machining phases, during which the workpiece is treated with different current intensities.
DE 42 27 005 A1 discloses a method for the electrochemical machining of a workpiece in which an abrupt rise in current takes place between at least two machining periods. Here, a direct current flows between a tool and a workpiece. This current may be supplied in either a continuous or a pulsed manner. The method described in DE 42 27 005 A1 is used in particular for the deburring of workplaces.
During a first machining period, the current intensity is chosen there to be relatively low, in order to prevent overheating of the electrolyte. An initial current intensity of 0.5 A per millimeter of overall edge length of the workpiece to be machined has been found to reliably accomplish this. The machining time during the first machining period is 10% to about 40% of the overall machining time, which is often 1 to 3 seconds. Following this first machining period, the operating current is increased abruptly by a factor of 1.3 to 10. This abruptly increased current level is retained in a second machining period up to the end of the machining process, it being possible for further increases in the current level to follow during the machining period. With the method described here, it is intended to reduce the formation of electrical short-circuits.
With the electrochemical machining methods known from the prior art, it is known however that, particularly when sodium nitrate is used as the electrolyte, deposits occur on the workpiece and may lead to a passivation of the workpiece surface. Moreover, during electrochemical machining with current densities that are too low, discolorations and great surface roughnesses occur on the surfaces of the workpiece. Since, however, a high surface quality is desired, relatively high current densities must be used, which in turn is detrimental to machining accuracy. Although it is possible to improve the surface quality by a pulsed direct current, this leads to a slowing of the machining process because, between the individual current pulses, appropriate current capacitances first have to be built up by a current source. Moreover, continuous applications of great current intensities require correspondingly dimensioned current sources, which cause high costs in acquisition and during operation.
The object of the invention is therefore to solve at least partially the problems described with respect to the prior art and, in particular, to provide a method for the electrochemical machining of at least one workpiece that allows a high machining rate, is inexpensive to use and achieves a high surface quality of the workpiece.